The invention relates to improvements in papermaking machines, and more particularly to improvements in a dryer section for drying a traveling fibrous web such as in a papermaking machine for improved handling of the web and improved drying.
In present high speed papermaking machines with improved formation and improved pressing, the web can be formed faster and better, particularly with the new generation of twin wire formers so that the wet end is no longer the limiting constraint to reaching higher productions. Therefore, the demand is increased for improvements in the dryer section of the papermaking machine to accommodate higher speeds. A primary limitation on lightweight grades, such as newsprint, is sheet flutter in the dryer section. At the beginning of the dryer section during the early stages of drying when the web contains more moisture and is still very fragile, it becomes slack and cross-machine tension variations cause wrinkles in the sheet. Disturbances in web travel can cause the sheet to fold over upon itself resulting in the formation of machine direction wrinkles. Most of these wrinkles occur near the front or back edges of the sheet and may vary in size from a few inches to several feet in length. In order to minimize slackness in the web and the resultant susceptibility to wrinkling, the draws can be tightened. This results in less fold-overs and less wrinkling, but it is done at the expense of increased wet and dryer sheet breaks.
In developments in improved dryer sections, one arrangement has involved a felt run known in the art as a "uno-run" or "single felt run" which has been installed on many machines producing lightweight grades. The uno-run is a dryer felt arrangement where a single felt is utilized to wrap both top and bottom dryers used usually only in the first dryer section. The paper web is then sandwiched between the felt and cylinder on the top dryer drums and travels on the outside of the felt as it goes around the bottom dryer drums. The purpose of this arrangement is to stabilize the sheet from edge fluttering and billowing while the web is weak and prone to breaks. However, a primary drawback of the single felting arrangement stems from the large loss of drying which results on the bottom cylinders where the web is on the outer surface of the felt and the felt, therefore, acts as an insulator and prevents effective heat transfer to the sheet.
An important object of the present invention is to provide an improved dryer section utilizing a single felt run, but avoiding the disadvantages of loss of drying when the sheet is carried on the outer surface of the felt over the drum and retaining good runnability which is associated with single felting and with the benefit of no drying loss on the bottom drums.
In accordance with the principles of the present invention, the traveling web or sheet enters on a small dryer or a baby dryer roll, travels around a second drum on the outside of the felt for heat conditioning, and is then sandwiched between the felt and cylinder on the No. 3 dryer drum which is above the No. 2 drum. The sheet is then conveyed on the felt through a series of substantially spaced stacks of drums, the drums being in vertical alignment in the stacks and being arranged so that the sheet is in direct contact with the drum surface on both the upper drum and the lower drum in each stack. To accomplish this, a turning roll is positioned between the drums which may be preferably in the form of a vacuum roll or a grooved felt roll, and arrangements are made with turning rolls positioned to obtain a wrap around the top and bottom rolls with the felt and sheet traveling directly across the spans from stack to stack.
With this arrangement, there are no open draws in the dryer driver group, and the web or sheet is always supported by the felt. With the exception of the advantageous arrangement wherein the sheet rides on the outer surface of the felt on the first bottom dryer roll, the sheet is always sandwiched between the felt and cylinder. There, therefore, is no loss in drying as is encountered with conventional single felt dryers where the sheet travels on the outside of the felt on the bottom dryer drums. Also, because of the large felt and sheet wrap angles, drying rates should be higher than on conventional dryers. Other advantages include more positive threading when the turning rolls are vacuum rolls. Also, the arrangement provides for self-dumping for all sheet runs. The arrangement accommodates any convenient height for the different dryer sections and as many vertically arranged drums can be utilized as are desired and as are accommodated by the mill overhead structure. The web is continuously controlled to eliminate flutter, and improved results from the relatively open relation between the roll stacks and providing for good moisture evaporation are obtained.
Other objects and advantages and features will become more apparent with the teaching of the principles of the present invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: